This invention relates to a protective circuit for protecting a push-pull type power amplifier, and more particularly to a protective circuit for a power amplifier which can decrease voltage impressed on a semiconductor element constituting the protective circuit.
An audio power amplifier used to date has the arrangement shown in FIG. 1. According to the prior art audio power amplifier, a load current-detecting resistor R.sub.E (R.sub.E =r.sub.E +r.sub.E) is connected between the emitter of a transistor of first conductivity type whose collector is connected to a positive power source +V.sub.CC and the emitter of a transistor of second conductivity type whose collector is connected to a negative power source -V.sub.EE. A switch S is connected between an intermediate point on the resistor R.sub.E and a load L (speaker SP), one end of which in grounded. A collector-emitter circuit of a transistor Q.sub.3 for detecting the resistance of the load L, for example, the short-circuiting thereof is connected through a resistor between the intermediate point on the resistor R.sub.E and the positive power source +V.sub.CC. The base of the transistor Q.sub.3 is connected to a junction of resistors Ra, Rb collectively constituting an attenuator. A coil 17 for operating the switch S is connected to the positive power source +V.sub.CC through a driving transistor Q.sub.4. The base of this driving transistor Q.sub.4 is supplied with an output from the collector of the transistor Q.sub.3. Where the load L is shortcircuited, the transistor Q.sub.4 is put into operation to open the switch S, thereby protecting the power amplifier. This protective operation is undertaken when the following formula is satisfied: EQU r.sub.E I.sub.0 -.alpha.v.sub.0 .gtoreq.V.sub.TH
where:
r.sub.E =resistance of the resistor r.sub.E of FIG. 1 PA1 I.sub.0 =load current PA1 .alpha.=coefficient defined mainly by the resistances Ra, Rb of the attenuator of FIG. 1 PA1 v.sub.0 =interterminal voltage of the load L PA1 V.sub.TH =threshold voltage of the transistor Q.sub.3, namely, a detecting level of the load resistance.
As apparent from the above formula, the switch S is immediately opened when the load is short-circuited, and the load resistance is substantially reduced to zero to allow the passage of large load current.
Where the protective circuit is integrated, it is necessary to use a semiconductor element of low withstand voltage and reduce the power consumption of the protective circuit. In this connection, the transistor Q.sub.3 of the prior art protective circuit of FIG. 1 is discussed below. The voltage applied between the collector and emitter of the transistor Q.sub.3 varies between the voltage +V.sub.CC of the positive power source and the voltage -V.sub.EE of the negative power source, therefore the withstand voltage BV.sub.CEO between the collector and emitter regions is required to have as large a value as V.sub.CC +V.sub.EE, where, therefore, the protective circuit is integrated, the withstand voltage of the transistor Q.sub.3 has to be raised to the above-mentioned high level. This means that a large chip has to be used, increasing the cost of an integrated protective circuit. Further drawbacks of the prior art protective circuit are that if high voltage has to be impressed on a semiconductor element, power consumption will rise, making it necessary to use a package of low thermal resistance; special means should be provided to promote heat dissipation; and the internal resistance of the semiconductor element itself should be decreased to admit of application of small operating current. Therefore, the arrangement of the prior art protective circuit shown in FIG. 1 can not be regarded as adapted for integration.
It is accordingly the object of this invention to provide a protective circuit for a power amplifier which can be formed of a semiconductor element having a relatively low withstand voltage.